1. Field of the Invention
The invention relates to a multi-channel amplifier circuit, and more particularly to a multi-channel amplifier circuit with a beat frequency cancellation circuit capable of cancelling beat frequency effects between different channels.
2. Description of the Related Art
As technologies advance, the variety of functions provided by portable electronic devices (such as mobile phones, personal digital assistant, tablet computer, or others) has been greatly increased. Among them, recently, multimedia services have become one of the fundamental features that are provided with portable electronic devices. In order to generate high quality audio signals, a power-saving and high efficiency power amplifier is required. A class D amplifier is a preferable choice for small-sized electronic devices since the circuit area of the class D amplifier is small, and amplifying efficiency of the class D amplifier can be over 90%. Therefore, the class D amplifier has replaced the class AB amplifier and has become main stream when applying audio power amplifiers.
The class D amplifier is also called a digital power amplifier, which modulates and amplifies input analog signals and generates digital amplified output signals. FIG. 1 shows a circuit diagram of a basic class D power amplifier, in which a pulse width modulation (PWM) modulator 101 modulates an input signal Vin to generate a digital signal. The digital signal is transmitted to the driver circuits 102 and 103, and amplified and output by the power transistors Q1 and Q2. Finally, a low pass filter 104 extracts the original audio signal at the input terminals and transmits the extracted signal to the loud speaker 105.
Because the inputs of the power transistors are digital signals, the power transistors Q1 and Q2 work in the saturation and cut-off states. For this reason, the power consumed by the power transistors Q1 and Q2 is very small, which not only increases overall efficiency of the power amplifier, but also allows the size of a heat dissipation device to be small. Hence, the size of the electronic components of a class D amplifier can be small. Moreover, while, in general, the amplifying efficiency of a class AB amplifier is only about 50%, the amplifying efficiency of the class D amplifier can be over 90%, and even approach 100%. Therefore, the class D amplifier is greatly used in the audio power amplifying fields.
However, for multi-channel audio signal amplifying applications, the different channel digital signals may have different oscillation frequencies after being modulated. The frequency differences between the audio signals in different channels cause a beat frequency effect. When the frequency difference falls to a frequency band perceptible by human ears, a beat tone may become noise heard by a user when there is no audio signal being input.
Therefore, the noise caused by the beat frequency in multi-channel audio amplifier is a problem desired to be solved.